This invention relates to a solid state corona discharger for use in corona chargers which charge and discharge photosensitive members in electrophotographic copying machines, ozone generators and the like.
Corona dischargers have been widely used in corona chargers which charge and discharge photosensitive members in recording devices utilizing an electrophotographic process, such as electrophotographic copying machines, facsimiles, laser printers and LED (light emitting diode) printers, and also have been widely used in ozone generators for preserved freshness of foods in a refrigerator, sterization, deodorization or decolorization, and clean-up.
To charge or discharge photosensitive members in copying machines, corotorons or scorotorons have been used so far in which a fine wire of several ten micrometers in diameter. is enclosed with a U-shaped plate electrode. But a fine wire with toner or paper powder attached or with flaw, unevenness or other some infinitesimal defects is likely to cause irregular distribution of charges. In particular, in the case of minus charge when the drawback remarkably manifests itself, improvements are made by a provision of scorotorons equipped with a screen over an opening in said plate electrode, thus resulting in additional drawbacks, such as larger, more complex, and more expensive construction.
With dischargers having said fine wire, the wire is troublesome in cleaning up and maintaining the dischargers. Yet further, industrial copying machines wide in size need a long wire, which may cause vibration, thus introducing additional drawbacks such as irregular charges and burnout due to abnormal spark discharge.
Aiming at depriving these drawbacks of such conventional finewire corona dischargers gave rise to an invention of charging and discharging device known as the solid state charger (hereinafter abbreviated as SSC) wherein firstly an alternating current or pulse-wise voltage generates ions and electrons over a dielectric-member surface and secondly a direct current electric field transfers them on a surface to be charged, some of which are proposed in U.S. Pat. Nos. 3,438,053 and 4,155,093, etc.
The concept of SSC seems to originate in an electrode construction and electrical circuitry means for electrostatic printer heads disclosed in U.S. Pat. No. 3,438,053 (applied for in July, 1964). FIG. 1 is the illustrative drawing of the patent. Paired electrodes 2 and 3 are separated with a dielectric member 1 and exposed to the air. On the paired electrodes a pulse source 6 applies a pulse voltage to produce ions together with electrons. DC power sources 7 and 8 generate a dc field in a space surrounded by the paired electrodes 2 and 3, a control electrode 5, and a surface 4 to be charged (working as an opposite electrode), and the field in turn transfers the ions to the surface 4 to be charged. Because the electrodes 2, 3, and 5 are all exposed to the air, the system is apt to produce abnormal spark discharge due to dust deposit, changes in environmental conditions and other external factors, thus suffering from the failure to build up an electrical field enough to yield a sufficient amount of ions.
To overcome the drawback, a method is used to control the charging and discharging of substances close to a surface layer by means of ions in the silently-discharging corona which is formed on the surface layer by a bank of electrodes in use for application of high ac voltage, arranged inside a dielectric member and on the surface layer.
Another method, disclosed in U.S. Pat. No. 4,155,093, is a combination of the generation and transfer of ions, aiming at utilization for both the electrode head of printers and the charger of copying machines. Contrary to said exposed electrodes, as shown in FIG. 2, this method uses electrodes 2 and 3, which are separated by a dielectric member 1, thereby remarkably increasing stability for spark discharge. Numeral 10 is a section where ions are formed and stored. Numeral 9 is a charging switch. An ac power supply 6 uses a voltage of sine, triangular or rectangular wave. With this method, however, the practical thickness of the dielectric layer 1 between paired electrodes 2 and 3 must be less than 100 micrometers and preferably as thin as less than 50 micrometers for better performance. Accordingly, the presence of initial defects in manufacturing, or foreign matters (dust) or pinholes, or other weak points in terms of voltage withstand ability in the dielectric member causes drawbacks that under high voltage being applied, large fluctuation appears in load capacity, and short-circuit takes place, resulting in operational failure, thus minimizing allowance and reliability. Even though protective resistances and capacitances are previously added to successfully come up with such possible failures, these parts are not only of costly high voltage resistance, but also actual operation of these parts needs larger resistance and smaller capacitance of said additional parts to effectively prevent breakdown damage, thus requiring a considerably higher ac output voltage than that actually required at the ion generating section, resulting in a larger and more expensive power supply.
Contrary to said SSC, a device as shown in FIG. 3 is known in which paired electrodes 2 and 3 entirely set in a dielectric member 1. But a dc electric field will affect the surface layer of the dielectric member 1 to be filled with reversed-polarity ions to the charged polarity ions, which may fail in desirable charging, thus resulting in a poor actual charge efficiency.